There is an increasing unmet need for immunoassays that yield a maximal amount of information from a minimal amount of patient sample. One way to address this challenge is to develop very highly multiplexed, sample sparing assays. This proposal presents new methods that integrate synthetic biology and next generation DNA sequencing (NGS) to characterize the specificities of humoral and cellular immune responses using genetically encoded antigen libraries. The multi-PI project builds upon a solid foundation, which has been established in recent years, for developing highly multiplex, sample sparing immunoassays. The platforms described in this proposal will be developed and tested on a new antigen library that encompasses the proteomes of all viruses known to infect humans (the human virome). Proof-of-concept studies confirm both the quality of this library, and its utility or developing sample sparing, highly multiplex antigen specificity assays. The following three Specific Aims have been developed to broadly analyze human immune responses to viruses, using an absolute minimal amount of sample. Specific Aim 1. Development of minimal human virome serologic assays Two novel sample sparing serologic assays are proposed: 1) a 384-well, simplified bacteriophage-NGS based assay, and 2) a rapid cytometric Luminex bead array assay. These technologies will comprehensively characterize anti-viral antibodies at low cost, and require less than a single microliter of blood. Specific Aim 2. Development of a minimal antigen library screening assay for cytotoxic T lymphocytes A new platform for profiling CD8+ cytotoxic T lymphocyte (CTL) specificities has been devised. The system employs lentiviral delivery of a genetically encoded antigen library to present MHC I-peptide complexes on autologous antigen presenting cells, followed by phenotypic library enrichment and NGS analysis. Specific Aim 3. Development of a minimal antigen library screening assay for T helper cells A new platform for profiling CD4+ T helper (Th) cell specificities has been devised. The system employs lentiviral delivery of a genetically encoded antigen library to present MHC II-peptide complexes on autologous antigen presenting cells, followed by phenotypic library enrichment and NGS analysis.